Smell

Gale Encyclopedia of Psychology
COPYRIGHT 2001 The Gale Group Inc.

Smell

The sense that perceives odor by means of the nose and olfactory nerve.

Olfaction is one of the two chemical senses: smell and taste . Both arise from interaction between chemical and receptor cells. In olfaction, the chemical is volatile, or airborne. Breathed in through the nostrils or taken in via the throat by chewing and swallowing, it passes through either the nose or an opening in the palate at the back of the mouth, and moves toward receptor cells located in the lining of the nasal passage. As the chemical moves past the receptor cells, part of it is absorbed into the uppermost surface of the nasal passages called the olfactory epithelium, located at the top of the nasal cavity. There, two one-inch-square patches of tissue covered with mucus dissolve the chemical, stimulating the receptors, which lie under the mucus. The chemical molecules bind to the receptors, triggering impulses that travel to the brain . There are thousands of different receptors in the cells of the nasal cavity that can detect as many as 10,000 different odors. Each receptor contains hair-like structures, or cilia, which are probably the initial point of contact with olfactory stimuli. Research suggests that the sensitivity of the olfactory system is related to the number of both receptors and cilia. For example, a dog has 20 times as many receptor cells as a human and over 10 times as many cilia per receptor.

The cribriform plate forms the roof of the nasal cavity. The olfactory nerve passes through openings in this bone and ends in the olfactory bulb, a neural structure at the base of the brain. From there, olfactory signals are diffused throughout the brain to areas including the amygdala, hippocampus, pyriform cortex (located at the base of the temporal lobe), and the hypothalamus . Olfaction is the only sense that does not involve the thalamus . Olfaction messages are especially intensive in the amygdala, a part of the brain responsible for emotions, which may help the unusual power of certain smells to trigger emotions and recollections based on memories from the past. Further, a person's reaction to smell is mediated by context. For example, the same smell present in body odor is responsible for the flavor of cheese. In the first case, the smell is perceived as negative, in the second, it is positive. In humans, olfaction intensifies the taste of food, warns of potentially dangerous food, as well as other dangers (such as fire), and triggers associations involving memory and emotion . Olfaction is an especially important sense in many animals. A predator may use it to detect prey, while prey may use it to avoid predators. It also has a role in the mating process through chemicals called pheromones, which can cause ovulation in females or signal a male that a female is in a sexually receptive state. Although the existence of human pheromones has not been verified, olfaction still plays a role in human sexual attraction, as well as in parenting. Mothers can usually identify their newborn infants by smell, and breast-feeding babies can distinguish between the smell of their mothers and that of other breast-feeding women. Researchers have also found that children are able to recognize their siblings by smell and parents can use smell to distinguish among their own children. However, as people age the sense of smell diminishes, especially for men. By age 80, many men have almost no ability to detect odors. The intensity of a particular odor is strongly affected by adaptation . Odors may become undetectable after only a brief period of exposure. The sense of smell also plays an important role in the discrimination of flavors, a fact demonstrated by the reduced sense of taste in people with colds. The enjoyment of food actually comes more from odors detected by the olfactory system than from the functioning of the taste system. The olfactory and gustatory (taste) pathways are known to converge in parts of the brain, although it is not known exactly how the two systems work together. While an aversion to certain flavors (such as bitter flavors) is innate, associations with odors are learned.

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Smell

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Smell

Smell, called olfaction, is the ability of an organism to sense and identify a substance by detecting tiny amounts of the substance that evaporate and produce an odor. Smell is the most important sense for most organisms. Many species use their sense of smell to locate prey, navigate, recognize and communicate with others of their species, and mark territory.

The sense of smell differs from most other senses (sight, hearing, taste, and touch) in its directness. We actually smell microscopic bits of a substance that have evaporated and made their way to the olfactory epithelium, a section of the mucous membrane in the roof of the nasal

cavity of the nose. The olfactory epithelium contains millions of odor-sensitive olfactory nerve cells that are connected to the olfactory nerves. Hairlike fibers on the end of each olfactory cell react to an odor by stimulating the olfactory cells to send a signal along the olfactory nerve to the brain, which interprets the signal as a specific smell.

Human versus animal smell

There is no doubt that many animals have a sense of smell far superior to that of humans. Most vertebrates (animals with backbones) have many more olfactory nerve cells than humans. This probably gives them much more sensitivity to odors. Also, the structure in the brain that processes odors (called the olfactory bulb) takes up a much larger part of the brain in animals than in humans. Thus, animals have a greater ability to process and analyze different odors. This is why humans use dogs to find lost persons, hidden drugs, and explosives—although research on "artificial noses" that can detect scent even more reliably than dogs continues.

Still, the human nose is capable of detecting over 10,000 different odors, even some that occur in extremely minute amounts in the air. Many researchers are considering whether smell does not play a greater role in human behavior and biology than has been previously thought. For instance, research has shown that human mothers can smell the difference between clothes worn by their baby and those worn by another baby only days after the child's birth.

Words to Know

Olfactory bulb: The primitive part of the brain that first processes olfactory information.

Olfactory epithelium: The patch of mucous membrane at the top of the nasal cavity that contains the olfactory nerve cells.

Olfactory nerve cell: A cell in the olfactory epithelium that detects odors and transmits the information to the brain.

Pheromone: Scent molecules released by animals that affect the behavior of organisms of the same species.

Scientists are only beginning to understand the role that smell plays in animal—and human—behavior. For example, animals release chemicals called pheromones to communicate danger, defend themselves against predators, mark territory, and attract mates. Some researchers have suggested that humans also may release pheromones that play a role in sexual attraction and mating—although this hypothesis has not been proven.

Current research

Olfactory nerve cells are the only nerve cells arising from the central nervous system that can regenerate (be formed again). Some researchers hope that studying regeneration in olfactory nerve cells or even transplanting them elsewhere in the body can lead to treatments for spine and brain damage that is currently irreversible.

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smell

smell
/ smel/
•
n.
the faculty or power of perceiving odors or scents by means of the organs in the nose:
a highly developed sense of smelldogs locate the bait by smell. ∎
a quality in something that is perceived by this faculty; an odor or scent:
lingering kitchen smellsa smell of coffee. ∎
an unpleasant odor:
twenty-seven cats lived there—you can imagine the smell! ∎ [in sing.]
an act of inhaling in order to ascertain an odor or scent:
have a smell of this.•
v.
(past and past part. smelled
or smelt
/ smelt/ )
1. [tr.]
perceive or detect the odor or scent of (something):
I think I can smell something burning. ∎
sniff at (something) in order to perceive or detect its odor or scent:
the dogs smell each other. ∎ [intr.]
have or use a sense of smell:
becoming deaf or blind or unable to smell. ∎ (smell something out)
detect or discover something by the faculty of smell:
his nose can smell out an animal from ten miles away. ∎
detect or suspect (something) by means of instinct or intuition:
he can smell trouble long before it gets serioushe can smell out weakness in others.2. [intr.]
emit an odor or scent of a specified kind:
it smelled like cough medicine|
the food smelled and tasted good |
[as adj. , in comb.] (-smelling)
pungent-smelling food. ∎
have a strong or unpleasant odor:
if I don't get a bath soon I'll start to smellit smells in here. ∎
appear in a certain way; be suggestive of something:
it smells like a hoax to me.PHRASES:smell blood
discern weakness or vulnerability in an opponent.smell a rat inf.
suspect trickery or deception.smell the roses inf.
enjoy or appreciate what is often ignored.smell something up
permeate an area with a bad smell:
he smelled up the whole house.DERIVATIVES:smell·a·ble
adj.smell·er
n.

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smell

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

smell, sense that enables an organism to perceive and distinguish the odors of various substances, also known as olfaction. In humans, the organ of smell is situated in the mucous membrane of the upper portion of the nasal cavity near the septum. It is made up of the olfactory cells, which are actually nerve cells that function as receptors for the sense of smell. The free ends of the cells project outward from the epithelial tissue in the form of numerous hairlike processes. These fibers are buried in the mucus that coats the inner surface of the nasal cavity and are stimulated by various odors. Nerve fibers extend from the olfactory cells to an area of the brain called the olfactory bulb. Any disturbance of the nasal cavity—such as the common cold—in which the olfactory hairs are covered with excess mucus or other material, interferes with the sense of smell. Most physiologists agree that although a substance must be volatile to be sniffed by the nose, it must subsequently be dissolved in the mucous lining of the nasal cavity to be smelled. It is also believed that there are only a few basic odors (perhaps about seven), and that all other odors are a combination of these. Attempts at classifying the so-called primary sensations of smell have not yet been successful. The sense of smell is not as strongly developed in humans as in many other vertebrates, particularly carnivores which employ olfactory organs to locate food and detect dangerous predators. To many invertebrates (especially insects) as well, smell is a highly developed sensory mechanism, necessary in obtaining food, in finding mating partners, and in recognizing other animals.

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smell

smell smell a rat begin to suspect trickery or deception.smell blood discern weakness or vulnerability in an opponent.smell of the lamp show signs of laborious study and effort; the reference is to an oil-lamp, and according to Plutarch the criticism was once made of the work of Demosthenes, ‘His impromptus smell of the lamp’, meaning that his speeches were written rather than spoken orations.smell the roses in North American usage, enjoy or appreciate what is often ignored.

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